readability

Computing the approximate (U.S.) grade level needed to comprehend a text using the Coleman-Liau index

The Coleman-Liau index is a "readability" test represented by the following equation:

index = 0.0588 * L - 0.296 * S - 15.8

Where L is the average letters per 100 words and S is the average number of sentences per 100 words within the text. With this algorithm we can determine the approximate reading level using the traits of the provided text.

Quick overview of how readability.c works (without diving into the helper functions):

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

// Function prototypes
int count_letters(string text);
int count_words(string text);
int count_sentences(string text);
float calc_index(string text, int letters, int words, int sentences);

int main(void)
{
    // Get user input for text to analyze
    string text = get_string("Text: ");

    int letters = count_letters(text);
    int words = count_words(text);
    int sentences = count_sentences(text);

    // Compute the Coleman-Liau index of the text
    printf("%i letter(s)\n", letters);
    printf("%i word(s)\n", words);
    printf("%i sentence(s)\n", sentences);
    if(round(calc_index(text, letters, words, sentences)) >= 16)
    {
        printf("Grade 16+\n");
    }
    else if(round(calc_index(text, letters, words, sentences)) <= 1)
    {
        printf("Before Grade 1\n");
    }
    else
    {
        printf("Grade %.0f\n", round(calc_index(text, letters, words, sentences)));
    }
}

Example:

~/readability/ $ make readability
clang -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wshadow    readability.c  -lcrypt -lcs50 -lm -o readability
~/readability/ $ ./readability 
Text: She had a wasp tattoo about an inch long on her neck, a tattooed loop around the biceps of her left arm and another around her left ankle. On those occasions when she had been wearing a tank top, Armansky also saw that she had a dragon tattoo on her left shoulder blade. She was a natural redhead, but she dyed her hair raven black.
262 letter(s)
65 word(s)
3 sentence(s)
Grade 7